The invention is directed to the control of indirectly heated gas turbines. Specifically, this invention is directed to a primary system of controlling the temperature of heated compressed gas entering the expander, and a secondary (or backup) system which includes a safety valve for instantaneous release of heated compressed gas to the atmosphere to prevent overspeed of the turbine.
Typical single shaft, indirectly heated gas turbines comprise a compressor for producing compressed gas, a gas-to-gas heat exchanger for indirectly heating the compressed gas to produce heated compressed gas, an expander for expanding the heated compressed gas, and a generator connected to the single shaft for producing electricity. The control system of single shaft, indirectly heated gas turbines is responsible for safe operation of the power generation plant from start-up to shutdown, and protection against all eventualities. The control system is required to be xe2x80x9cfail-safexe2x80x9d, and shut the plant down safely upon the occurrence of any dangerous condition.
In conventional gas turbines the gas is heated by combustion of fuel directly in the compressed gas. This is called Internal Combustion. In an indirectly heated gas turbine, the gas is indirectly heated in a heat exchanger by conduction of heat through membrane walls. The membrane walls are heated by hot gases produced in a separate process, such as external combustion at near atmospheric pressure or such as exhaust gases from an industrial process.
The power of both conventional gas turbines and indirectly heated gas turbines is modulated by control of the temperature of the hot gas entering the expander. In a conventional (internal combustion) gas turbine, modulation of the hot gas entering the expander is achieved by modulation of the combustion of fuel. However, to change the temperature of compressed gas in an indirectly heated gas turbine, an adjustment of the heat input to the heat exchanger, as well as the temperature of the tubes within the heat exchanger needs to change before the temperature of the gas entering the expander will change. The internal components of the heat exchanger, such as the tubes and their supports, heat up and cool down slowly. Thus it is not possible to control the temperature of the heated gas of an indirectly heated gas turbine sufficiently fast for normal power swings, or in an emergency, by control of the heat input to the heat exchanger.
As is normal for all turbo machinery there are two separate control systems to protect against one of the systems developing a fault. The security of two systems is required to protect personnel and the equipment against catastrophic failure of the turbine by overspeed should one of the control systems fail. Two systems are required to meet code, to obtain insurance and avoid onus in litigation in the most unlikely event of failure. These systems must provide sufficiently fast adjustment of the temperature and/or flow of the heated, compressed gas entering the expander to compensate for sudden changes, minor and major, in load, and must also provide a fail-safe means of instantaneous shut off in the event of an overspeed of the turbine.
The inventive control systems for control of indirectly heated gas turbines are responsible for safe operation of the plant from start-up to shutdown, and protection against all eventualities. The control system is required to be xe2x80x9cfail-safexe2x80x9d, and shut the plant down safely upon the occurrence of any dangerous condition. In the proposed system all normal operation, including emergency shut down is managed by a primary, or #1 Control System. The secondary, or #2 Control System, functions to stop the plant in an emergency should the #1 Control System malfunction.
The #1 Control System controls system gas temperature and power output by modulating a flow of unheated compressed gas which flows through a bypass duct and valve which bypasses the heat exchanger and mixes with the heated gas leaving the heat exchanger to produce a lower mixed temperature entering the expander. In this way, the temperature of the gas and the power output can be changed rapidly.
The #1 Control System may also include additional means of quickly reducing or stopping the power of the indirectly heated gas turbine. An additional valve, referred to as a blocker valve, may be installed in the outlet of the heat exchanger. By closing the blocker valve, the flow through the heat exchanger is resisted, causing more unheated gas to flow through the bypass duct. This will further reduce the temperature of the gas entering the expander and obtain a faster reduction of power than can be produced by the bypass duct and valve alone, as may be required for overspeed protection.
The #2 Control System provides a second means of overspeed prevention, and includes a safety valve to instantly discharge to atmospheric pressure approximately half of the hot compressed gas upstream of the expander. The remaining half of the hot compressed gas will continue to pass through the expander and produce power but not enough to accelerate the indirectly heated gas turbine, thus preventing overspeed. For purposes of this invention, xe2x80x9csafety valvexe2x80x9d is defined as a device that is responsive to the speed of the turbine as opposed to responsive to pressure provided by a gas, steam, or a liquid. The safety valve includes a rupture disk that is critical to the operation of the inventive safety value, and comprises a frangible membrane clamped between parallel flanges within the ducting, and further includes a dagger assembly for rupturing the frangible membrane. The dagger assembly is actuated using compressed gas generated within the turbine.